The present invention is directed primarily to improved apparatus and method for the production of sulfuric acid although it also is the case that from the standpoint of removal of pollutants from waste industrial gases highly significant improvements in effectiveness have been obtained as compared with the disclosure in U.S. Pat. No. 3,273,316.
In the practice of the invention of U.S. Pat. No. 3,273,316 the conditions are not such as to be best suited for the production of sulfuric acid although some sulfuric acid may be inherently produced. However, such production is not mentioned in the patent and the conditions disclosed in said patent are not favorable to the production of sulfuric acid since the ice crystals are formed while in an atmosphere provided by the waste gases themselves which are low in oxygen as the result of having been used in the burning of the combustible material used as a fuel. The oxygen content in ordinary atmosphere is 20.95%, whereas in a typical waste industrial flue gas the oxygen becomes reduced to approximately 5%.
The carrying out of the cryogenic process for the production of sulfuric acid in accordance with the present invention is predicated on the following formulae: EQU SO.sub.2 + H.sub.2 O = H.sub.2 SO.sub.3
and EQU H.sub.2 SO.sub.3 + O = H.sub.2 SO.sub.4
the improvements of the present invention utilize not only the phenomenon that the gaseous pollutants in waste gases are more effectively absorbed at low temperatures but also the phenomenon that the aforesaid reactions, and more especially the reaction whereby H.sub.2 SO.sub.3 reacts with oxygen to produce H.sub.2 SO.sub.4, is promoted by using cryogenically chilled ice produced so as to be high in entrained oxygen. While most gases and other foreign materials are rejected upon conversion of water to ice, oxygen, under ice-forming conditions in an independent ice-making machine, becomes entrained to the extent of about 13.4% or more which supplements the oxygen content of the air. This entrained oxygen is released when the oxygen-containing ice crystals are melted while in contact with the waste combustion gases so as to be available for the final reaction to produce the sulfuric acid under the favorable conditions for said reaction provided by the low temperature. The principal aspect of this invention, therefore, involves the utilization of the aforesaid phenomena in combination whereby ice crystals formed by an independent ice-making machine so as to favor high oxygen entrainment are cryogenically supercooled and contacted with the waste gases, thereby not only promoting the taking up of waste gas pollutants by the use of low temperatures but also promoting the conversion of any sulfur dioxide to sulfuric acid by reaction with a high content entrained oxygen released during melting of the ice crystals. By this invention, therefore, highly effective method and apparatus are afforded not only for effectively removing pollutants from waste combustion gases but also for providing a new and highly significant improvement in the conversion of the sulfur dioxide content of such gases into sulfuric acid.
It also is a feature of preferred practice of this invention that the capacity to convert the sulfur dioxide into sulfuric acid may be still further improved by the enhancement of the entrained oxygen as produced by an independent ice-making machine by the employment of oxygen.
It is a further feature of preferred practice of this invention to include a small amount of manganese sulfate in the ice crystals, thereby promoting the solubility of sulfur dioxide in water produced by the melting of the ice with resultant increased conversion to sulfuric acid by reaction with oxygen liberated from the ice crystals.
This invention whereby the ice crystals are separately formed under conditions favorable to oxygen entrainment followed by cryogenic chilling and contact with waste combustion gases also is of advantage in that it lends itself to better distribution of ice crystals in the ice filter bed that is contacted by the waste gases. In practicing the invention disclosed in U.S. Pat. No. 3,273,316 the ice crystals are formed primarily in the region below the ice filter bed formed therefrom and are carried upwardly as they are formed in the downwardly projected water spray by entrainment with the upward flow of waste gases so as to collect on the underside of the lowermost of the baffle supports for holding the ice crystals in place with passage of some of the ice crystals through the central opening in the lowermost baffle support for continued travel to the baffle support means higher up in the column. While this system is effective in the removal of pollutants, nevertheless by forming ice crystals in a separate ice-making machine and by conveying them to the filter bed means so as to be disposed on top of a suitable foraminous support for the ice filter bed better distribution of the ice crystals is provided for effective contact with the waste gases with resultant improved conditions for the production of sulfuric acid. Further in this connection, it is to be noted that by supplying cryogenically chilled ice crystals formed by means of an independent or separate ice-making machine as distinguished from injecting a liquid cryogenic material into a downward spray of water as disclosed in U.S. Pat. No. 3,273,316 the foraminous support for the ice crystal filter bed is permitted to extend completely across the chamber traversed by the waste gasses, thereby providing positive control whereby all the waste gases must pass through the ice filter bed with good distribution thereacross. A further advantage resulting from the use of an independent ice-making machine is that a greater surface area of ice crystals for contact with the waste gases is made possible with resultant enhanced output of sulfuric acid due to the fact that the improvement of the present invention is not subject to the design limitations that are imposed when ice crystals are formed by the injection of a cryogenic liquid into a downwardly directed spray of water.
It is of further advantage in the practice of the present invention that better control of ice crystal size is possible. When ice crystals are formed by injecting a cryogenic liquid into a downwardy directed spray of water as disclosed in U.S. Pat. No. 3,273,316 the ice crystals that are formed are relatively small and may be compared with snow in that they are sufficiently small to be carried upwardly by the waste gases so as to become collected on the underside of the baffle supports for the ice filter bed means. Under these conditions a certain amount of bridging between the ice crystals occurs which interferes with travel of the waste gases through the ice bed. By initially forming the ice crystals in an independent ice-making machine the size and shape may be better controlled. While some bridging may occur between such preformed crystals, it is much less than that which occurs when using the method and apparatus disclosed in U.S. Pat. No. 3,273,316, thereby obtaining better contact with the waste gases and correspondingly increased effectiveness in the production of sulfuric acid.
The production of sulfuric acid is further improved as compared with utilization of the method and apparatus disclosed in U.S. Pat. No. 3,273,316 by recirculating the waste gases which have been chilled by passage through the ice filter means so as to pass through a jacket which surrounds the chamber below the ice filter bed so as to effectively chill the inner surface of this chamber with which the incoming gases make contact. This is significant not only from the standpoint of conserving cryogenic material but also from the standpoint of decreasing the buoyancy of the gases with attendant more effective separation of impurities therefrom, including the removal of sulfur dioxide and its conversion to sulfuric acid.
One of the highly significant practical advantages of the method and apparatus of the present invention as compared with the disclosure in U.S. Pat. No. 3,273,316 is that great savings are made possible as regards the amount of cryogenic material that is used. While both employ a cryogenic material, such as liquid nitrogen, the calculated amount of cryogenic material which is used in the practice of the present invention is only about half the amount that is employed when utilizing the invention disclosed in U.S. Pat. No. 3,273,316 for treating a corresponding amount of waste combustion gases. As applied to the production of sulfuric acid, this means that for a given amount of liquid nitrogen, for example, the amount of waste gases treated may be roughly doubled as compared with that which occurs in the practice of the invention disclosed in U.S. Pat. No. 3,273,316. This is highly significant from the standpoint of economics in a commercial operation since the supply of cryogenic material is the principal cost consideration.
The apparatus of the subject invention for recovering sulfuric acid from industrial waste gases containing sulfur dioxide so as to include the several improvements of this invention comprises a waste gas purifier of the cryogenic type, and a cylindrical insulated jacket spaced from the gas purifier thereby defining a passageway; means for cirulation waste gases from the output or upper end of the waste gas purifier to the lower end of said passageway; separate means for producing ice for input to the waste gas purifier; and cryogenic means for injecting cryogenic materials into the ice preferably prior to its admission to the waste gas purifier. In operation, as the waste gases are passed through the purifier, the sulfur dioxide therein is separated from the waste gases and reacts with the water and oxygen within an ice filter portion of the waste gas purifier to produce sulfuric acid that is collected at the lower end of the waste gas purifier. The remaining gases are circulated through the top of the purifier, through the circulating means, and then through the passageway preparatory to exhausting to the atmosphere. As the cooled waste gases pass through the passageway they function as a thermal dynamic drive for cooling the waste gases passing through the industrial waste purifier. The method of the subject invention for recovering sulfuric acid comprises passing the waste gases through a waste gas purifier of the cryogenic type so as to expose the sulfur dioxide to tn enriched oxygen atmosphere and water resulting from liberation of oxygen entrained in the ice for producing sulfuric acid.
In obtaining the several improvements of the subject invention, the process of the subject invention operates in conjunction with the apparatus in such manner that as the waste gases enter into the industrial waste gas purifier, the gases are chilled down by a spray of water that is at a temperature close to freezing and is pressurized. The descending water spray effectively forces the ascending hot waste gases against the interior walls of the vertical extending column of the industrial waste gas purifier, whereby the gases are contacted with the interior walls of the column. The frictional contact of the industrial waste gases against the wall effectively ionizes the gases. Ionization of the gases also is effected because of the friction of the particulate matter within the gases, as well as the changes of density due to the temperature variation of the industrial waste gas as it ascends and is simultaneously cooled, and also by the condensation of the water vapor in the industrial waste gas. Ionization of the waste gases is important in that it greatly aids in the creation of precipitation within the purifier. This is of particular importance with respect to the first of the aforesaid two formaulas. As background, it is known that in the scrubbing of sulfur dioxide from flue gases using industrially avaiable water, the apparent solubility is greater than indicated from the equilibrium values of sulfur dioxide with pure water. This excess solubility indicates that a secondary reaction takes place after the flue gases dissolve or interact with the alkaline impurities, or with dissolved oxygen which results in the formation of sulfuric acid. It is known that some metallic ions act as catalysts for absorbing sulfur dioxide by water. For example, manganese sulphate, in concentrations as low as 0.0028 p.p.m., increase the solubility of sulfur dioxide in water by 600%. Hence, when a mixture of sulfur dioxide in air is bubbled through water containing manganese ions as a catalyst, a 30% sulfuric acid concentration is obtained. In order to obtain the same effect as the provision of manganese sulphate, the subject invention provides an arrangement wherein the flue gases are ionized upon entering the waste gas purifier in order to increase the solubility of sulfur dioxide with water. As previously noted, ionization of the gases is achieved by forcing the waste gases in frictional contact with the side walls of the purifier, thereby causing the waste gases to become ionized during their ascent through the ice bed portion of the purifier.
Another important aspect of the subject invention is the second formula cited above as part of the subject cryothermal process, and in particular, the subject means for providing an enriched oxygen atmosphere to the ascending industrial waste gases as they pass through the subject apparatus. As part of the subject apparatus, the industrial gases are caused to pass through an ice bed or ice filter means, and means are provided for insuring the inclusion of a sufficient amount of oxygen within the ice bed for enabling the reaction of H.sub.2 SO.sub.3 and oxygen to form sulfuric acid. More particularly, the subject apparatus includes means for insuring the inclusion of oxygen molecules within the crystals making up the ice bed. As is known, when ice is formed, the gaseous inclusions found in it originate either from the atmosphere or from the freezing of water. Thus, when water freezes, gases dissolved in it separate completely out of the solution and form bubble inclusions with a new quantitative ratio of the gases. The self-generating or autogenous air of congelation ice formed from completely fresh water will be entrained by oxygen to the extent of 13.4%, whereas carbon dioxide will be found to the extent of 1.7%. Consequently, the entrained gas will be correspondingly deprived of nitrogen and the inert gases. Since the percentage of oxygen in the air (atmospheric air) is 20.95%, to this must be added the enrichment percentages of oxygen found in ice when formed. These percentages will vary depending on the particular environmental conditions existing, such as the temperature of the ice, pressure, etc., and have been measured in different locations as follows: Alpine glacier ice has 22.7% oxygen in the air entrained in ice; Spitzbergen glacier ice, 20.4%; and Barents Sea 24.0 - 26.3%. In addition it should be noted that various other parameters influence the amount of a foreign body within an ice crystal, such as the rate of crystallization, the size of the foreign body, the magnitude of the forces of surface tension between the body, the liquid and the ice crystal. In addition, the rate at which the ice freezes also controls the extent of the inclusions in the ice.
In the subject invention, oxygen enrichment of the ice is achieved by means of an ice-making machine, which produces ice crystals that are to be conveyed to the ice filter means or ice bed portion of the waste gas purifier. The ice-making machine is provided with fresh water, and oxygen in order to produce oxygenated ice crystals. While the crystals are being transferred from the ice-making means to the ice bed, super-cooling of the commercially manufactured ice crystals is accomplished by the spraying of the crystals with a cryogenic material. The chute is pressurized with either pure oxygen or compressed air to aid in the conveyance of the supercooled crystals to the ice bed.